The present invention relates generally to fiber optic temperature sensing in well bores and, more particularly, to a device and method for calibrating a Distributed Temperature Sensing (DTS) fiber deployed for same.
Distributed Temperature Sensing (DTS) is one method of monitoring temperature along the length of a well bore. DTS employs an optical fiber as both the communication line and the temperature sensor. A fiber optic cable is installed along the length of a well. A laser or other light source at the surface of the well transmits a pulse of light into the fiber. The light pulse excites atoms as it propagates through the fiber, causing the stimulated atoms to, among other activity, reflect detectable light back towards the surface for detection. The frequency of the reflections relative to the pulsed light are shifted in accordance with the temperature of the atoms along the fiber. These reflections are processed as a function of time to derive temperature as a function of well depth, with earlier reflections indicating the temperature at relatively shallow depths, and later reflections indicating the temperature at relatively deep depths. Such time-to-depth conversion is possible because the speed at which light travels through the fiber is known. Temperature may be derived from the reflections by computing the ratio of intensities between selected wavelengths in the reflections (e.g., through Raman back scattering analysis). Raman back scattering analysis is discussed, for example, in U.K. Patent Application 2,140,554, published November, 1984, which is hereby incorporated by reference in its entirety. Through systematic pulses, the processor is able to monitor temperature along the entire length of the fiber. Hence, the optic fiber acts as a temperature sensor, permitting the reading of temperature gradients and changes throughout the well.
However, DTS fiber may degrade. The most common reason why DTS fiber degrades is the absorption of hydrogen. Although DTS fiber is individually cladded and may be hermetically sealed within a bundle of fibers in a cable, hydrogen is eventually absorbed into the DTS fiber. Hydrogen causes the fiber to xe2x80x9cdarken.xe2x80x9d Hydrogen in glass absorbs light, turning it into heat and thus attenuating the light. Such degradation is aggravated by temperature.
Degradation of DTS fiber may yield unpredictable (incorrect) temperature measurements. As DTS fiber degrades, the rates of change in the intensity of reflections at different wavelengths are neither uniform nor predictable. Moreover, DTS fiber is not guaranteed to degrade uniformly. Conditions vary along the length of a borehole and, therefore, along the length of the DTS fiber. While conditions near the surface may be hospitable, at depth the temperature may reach 200 degrees Celsius, accompanied by pressure of 15,000 psi (pounds per square inch).
Heretofore, corrupted DTS fiber has been unwittingly used or discarded and replaced if it is discovered to be the culprit in producing corrupt data. However, extracting and installing fiber optic cable is costly. Moreover, extracted cable is generally not reusable.
A further problem with the deployment of extremely long DTS fiber is noise. The longer the fiber, the greater the noise. This can be visualized on a plot of temperature versus length. As the length of the fiber increases (i.e. as the distance from the light source and light detector on the surface increases), the plotted temperature becomes progressively more jittery.
Thus, there exists a need to detect the degree of degradation of DTS fiber and/or noise and to adjust for it, i.e., to calibrate the DTS system. Such an improvement would add greater confidence in accuracy and reliability of a DTS fiber.
The present invention is directed to overcoming, or at least alleviating, one or more of the problems set forth above.
A method and apparatus for calibrating a DTS system. One or more discrete temperature sensors are positioned adjacent to a DTS fiber to calibrate data generated from DTS fiber. The discrete temperature sensors preferably comprise FBG (Fiber Bragg Grating) sensors.